The Effects Of Different Grazing Intensity On The CO₂ Flux In The Alpine Meadow

Since the Industrial Revolution, the concentration of greenhouse gas in the atmosphere is continuing to increase leading to the increase of the global greenhouse effect. This situation has attracted considerable attention among scientists. Carbon dioxide（CO₂） is one of the most important greenhouse gas and its emission has increased from 6.1±0.3 Pg C year-1 in 1990 to 9.5±0.5 Pg C year-1 in 2011. CO₂ concentrations have increased by 40%since pre-industrial times. Although most of the CO₂ emissions come primarily from fossil fuel emissions and secondarily from net land use change emissions but in the nature ecosystem, grasslands play an important role in the total carbon exchange between the biosphere and the atmosphere. Grasslands are one of the most widespread ecosystem on the earth which cover nearly 20% of the Earth’s land surface. The Qinghai-Tibetan Plateau is the highest grassland plateau in the world with situating at 3500 m or more above sealevel（a.s.l.） and it covers an area of approximately 2.5 million km2 occupied approximately 25%of the total Chineseterritory and has a typical plateau continental climate. One of the main ecosystem types on the Qinghai-Tibetan Plateau is alpine meadow which covers nearly35% of the entire plateau. On account of the vast expanse and high fragility to environmental disturbances, the alpine meadows are not only regarded as a sensitive trigger of climate change in the Asian monsoon region, but also show pronounced feedbacks to human activities. The major land use of alpine meadows is livestock grazing, which is one of the most prevalent land uses in the region and has a great effect on carbon emissions by changingnutrient pool, plant community composition and vegetation cover, soil organic carbon, soil moisture and soil temperature. Therefore, it is necessary to explore the grazing effect on the grasslands especially on the alpine meadows.By far, the majority of related studies were implemented at relatively low elevations. In China, the most of related studies concentrated in the Inner Mongolia. Of course, a few researches were conducted in the alpine meadows. Although there were so many studies about the relationship between CO₂ flux and grazing, but a consistent conclusion has not been obtained and rarely focus on the CO₂ flux response to the grazing intensitiesand the length of time. Therefore, we carried out two three years experiments to observe the CO₂ flux response to the different grazing intensities. The two experiments include the experiment of yak and the experiment of sheep in the alpine meadow.The main objectives of this paper were to（1） estimate the value of the CO₂ flux under different grazing intensity in alpine meadow on the northeastern Qinghai-Tibetan plateau;（2） explore the effects of different grazing intensity on the CO₂ flux and soil properties; and（3） confirm the environment factors controlling the CO₂ flux.After three years observation, the main results are as follow:（1） In the experiment of yak: the mean annual cumulative CO₂ flux were 470.31,482.71, 462.16, 431.44 g C m-2 y-1 under non-grazing（Z）, light grazing（L）, moderate grazing（M） and heavy grazing（H）, respectively. The whole average was 461.66 g C m-2y-1.In the experiment of sheep: the mean annual cumulative CO₂ flux were 487.66,430.11, 434.78 g C m-2y-1under light grazing（L）, moderate grazing（M） and heavy grazing（H）, respectively. The whole average was 450.85 g C m-2 y-1.（2） In the two experiments, the CO₂ flux in the growing seasons was significantly more than that in the non-growing seasons. In the experiment of yak: the CO₂ flux in the growing seasons accounted 68.02%⁷4.36% for annual cumulative CO₂ flux. In the experiment of sheep: the CO₂ flux in the growing seasons accounted 69.34% 77.21% for annual cumulative CO₂ flux.（3） There were no significant relationship between different grazing intensity and the CO₂ flux in neither the experiment of yak nor in the experiment of sheep.（4） Temperature in soil surface, 5 cm depth and 10 cm depth showed a positive correlation with the CO₂ flux but not more significantly than Month cumulative precipitation. Month cumulative precipitation exhibited a significantly positive correlation with the CO₂ flux.